This invention relates generally to an improved system and method for cleaning of coal. More specifically, this invention relates to improved separation of carbon-based or organic combustible particles or matter comprising vegetation and the like, from suspended and substantially noncombustible inorganic particles or matter, such as clay particles and the like.
Coal deposits are generally defined as carbonized vegetation matter which, due to the effects of heat and pressure over a prolonged period of time, becomes compressed into a rock-like dark material which is combustible to provide a common fuel source used widely in various industrial applications, such as electrical generation plants and the like. In use, the coal is mined from the earth, and typically reduced in size as by grinding for subsequent combustion within a large firebox or the like. In a typical electrical generation facility, the heat generated by the combusted coal is used to heat water sufficiently to provide a source of steam used to drive appropriate steam-powered generators.
In a raw or as-mined state, coal deposits normally include a minor proportion of noncombustible inorganic matter, particularly such as clay particles, ash, sand, rock fragments, and the like, which are mixed into the carbon-based organic matter. This inorganic matter occurs naturally in the course of primordial formation of the coal deposits, due to sporadic flooding and other natural phenomena which inherently combines such inorganic matter with the organic matter. Upon subsequent coal combustion, this entrained or embedded inorganic matter is substantially noncombustible, and thereby melts within the firebox to rob heat from the combusted organic coal particles. While such inorganic particles can be removed from post-combustion flue gases by means of electrostatic precipitation or the like, there is nevertheless a significant reduction in the total combustion output energy of the combusted organic coal particles.
Mercury particulate and the resultant flue gas contaminant represent an especially problematic inorganic constituent in some mined coals. In recent years, governmental regulations have applied pressure to the coal industry to effectively remove such mercury particulate from mined coal, prior to combustion in a firebox.
In the past, a variety of systems and methods have been proposed for separating the noncombustible inorganic matter or particles from the combustible organic carbon-based coal matter or particles. Such systems and methods have generally comprised initial crushing of mined coal to a relatively small and preferably powder-like constituency, followed by passing the small powder-like coal through a vibratory conveyor for recovering separated inorganic minerals from the otherwise combustible carbon-based coal component. More recently, improved systems and methods have used ultrasonic vibration. See, e.g., U.S. Pat. No. 4,741,839 which discloses a ground coal powder delivered as a water-borne slurry along a descending vibrator tray activated by multiple ultrasonic transducers used to separate the inorganic matter from the carbon-based organic matter. Different densities of the physically separated but still inter-mixed inorganic particles vs. organic particles permits subsequent settling and/or centrifugal separation to recover the valuable organic matter for drying, and subsequent combustion, as well as removal of an undesired inorganic component.
The descending ultrasonically vibrated conveyor, however, requires pre-crushing or pre-grinding of mined coal chunks substantially into a powder form to provide the requisite water-borne slurry. Such grinding of the mined coal chunks to a powder form is a mechanically intensive process, with the incumbent wear and maintenance/replacement of components.
There exists, therefore, a significant need for an improved ultrasonic system and method for separating mined coal into noncombustible inorganic matter and combustible organic matter, prior to supplying the comparatively softer organic matter to a firebox for combustion, while substantially reducing and/or eliminating the maintenance-intensive process of pre-crushing or pre-grinding the mined coal in preparation for ultrasonic separation. The present invention fulfills these needs and provides further related advantages.